In recent years, an organic photoconductors containing an organic electrically conductive substance has been employed for an electrophotographic photoconductor (which is hereinafter also denoted as a photoreceptor). Such an organic photoconductor is advantageous, as compared to other photoconductors, since it is easy to develop materials responsive to various light sources inclusive of visible light to infrared light, can choose a material exhibiting no environmental pollution and is also low in production cost. Further, an organic photoconductor is also superior in electrostatic-charging property and potential retention property, which is advantageous for high-precision and high resolution required in recent digitization.
However, an electrophotographic photoreceptor, which is directly subject to external electrical or mechanical forces in the processes of electrostatic-charging, exposure to light, development, transfer and cleaning, is required to be durable, capable of maintaining charge stability, potential retention, and the like.
Specifically, in the recent trend of digitization, requirements for images of high-precision and high image quality are increased, and a small-particulate toner produced by a process of polymerization, such as a solution suspension toner or an emulsion polymerization coagulation toner, becomes mainstream. Such a small-particulate toner exhibits enhanced adhesion to the photoreceptor surface, producing problems such as insufficient removal of residual toner adhered to the photoreceptor surface after completing the transfer step. A broadly available cleaning system by using a rubber blade (which is hereinafter also denoted as blade-cleaning system) often causes phenomena such as “toner slippage” in which toner particles pass under the blade, “blade torsion” in which a blade is reversed, and occurrence of frictional noise between the photoreceptor and the blade, so-called “blade noise”. To overcome the foregoing “toner slippage”, it is required to increase the contact pressure of the blade against the photoreceptor but its repeated use produces problems such that, specifically in an organic photoreceptor, the surface is abraded, resulting in deteriorated charging performance. There was also required sufficient durability to deterioration due to ozone or nitrogen oxide produced at the time of charging but a problem regarding this phenomenon arose specifically in organic photoreceptors.
Based on the foregoing background, there was proposed a technique of providing a surface layer on the photoreceptor surface to achieve enhanced mechanical strength. Specifically, there was proposed a technique of preparing a photoreceptor of enhanced durability to counter the surface abrasion or flaws due to friction by a cleaning blade, in which a compound (monomer), also generally called a polymerizable compound, was coated onto the surface layer of a photoreceptor, as described in, for example, JP 2009-080401A and JP 2009-069241A.
There was also proposed a technique in which inorganic particles such as silica were dispersed onto the surface layer to achieve enhanced mechanical strength, as described in, for example, JP 2009-069541A and JP 2002-333733A.
The foregoing prior art achieved enhanced hardness of the photoreceptor surface with a polymerizable compound to attain enhanced resistance to flaws and abrasion to obtain a photoreceptor of enhanced durability. However, there were used commercially available tri- or tetra-functional monomers as a polymerizable compound but they did not sufficiently performed not sufficiently.
To achieve enhanced abrasion resistance of an organic photoreceptor was proposed a photoreceptor which was provided with a curable protective layer formed of a radical-polymerizable compound and a radical-polymerizable compound capable of transporting electrons, as described in JP 2009-251140A. However, the photoreceptor disclosed in JP 2009-251140A introduced a charge transport group, which was also capable of causing steric hindrance in the resin structure of the protective layer, disturbing development of the cross-linking structure, rendering it difficult to achieve sufficient abrasion resistance, and unreacted groups remaining in the protective layer easily causing image blurring.
JP 2009-251140A also described the combined use of a tri-functional radical-polymerizable compound and a penta- or hexa-functional radical-polymerizable compound. However, the combined use of such radical-polymerizable compounds was based on the use of a radical-polymerizable compound exhibiting charge-transporting capability, so that the protective layer formed of such a combined composition did not solve problems including abrasion resistance, flaw resistance, image blurring and the like. Further, when forming a protective layer, the tri-functional radical-polymerizable compound permeates into a lower photosensitive layer, causing cracking of the photosensitive layer; however, there is no description with regard to a preventive measure to permeation into the lower layer.